An electronically controlled automatic transmission for automotive vehicles has clutches and brakes for establishing and disestablishing torque flow paths through multiple-ratio gearing from a vehicle engine to vehicle traction wheels. A hydrokinetic torque converter is disposed between the engine and torque input elements of the gearing. The neutral idle control system of the invention is particularly adapted to be used in an automatic transmission control system of the kind disclosed, for example, in U.S. Pat. No. 5,389,046. A multiple-ratio gearing system for the transmission of the '046 patent is disclosed in U.S. Pat. No. 4,938,097.
Automatic transmissions having a torque converter and multiple-ratio planetary gearing, such as the transmissions disclosed in the '046 and '097 patents, do not have the ability to provide a neutral idle since the forward and reverse clutches are applied with line pressure. With the engine throttle closed, the line pressure for the control valve system is reduced, but the minimum value is relatively high (e.g., 50 psi). The forward drive clutch or the reverse clutch thus is fully engaged when the engine throttle is closed as the torque converter continues to deliver torque.
Attempts have been made to improve the driveline efficiency by unloading the converter when the engine is idling and the vehicle is at rest. Such control valve systems make it possible to reduce undesirable noise vibration and harshness in the driveline when the engine is idling and the automatic transmission is in a drive mode. U.S. Pat. No. 5,272,630 is an example of an attempt to avoid the noise vibration and harshness problem.
The '630 patent describes a control strategy for supplying a residual or threshold pressure to the fluid pressure actuators for the clutches at the beginning of the acceleration mode using an electronic, closed-loop controller. The actuators can be fully stroked at the instant the acceleration mode begins. The closed-loop controller compensates for changes in driveline variables such as changes in oil temperature and clutch spring forces, for example, so that each clutch engagement is repeatable.
The control system of the '046 patent, unlike the control system for the '630 patent, lacks the ability to provide neutral idle since the forward and reverse clutches are applied with a minimum value that is relatively high at the initiation of the acceleration mode. For that reason, the clutches remain applied when the engine idles and the vehicle is at rest.
The neutral idle feature described in the '630 patent requires the addition of extra solenoids, pressure accumulators, and extra valve elements in the control system, which adds complexity and manufacturing cost to the transmission compared to the control system of the '046 patent.
U.S. Pat. Nos. 4,938,097, 5,272,630 and 5,389,046 are owned by the assignee of the present invention.